1. Field of the Invention
This invention relates in general to a linear power supply control circuit that operates (changes preregulator voltage) in response to changing input line or system load requirements, and, more particularly, to a control circuit which automatically switches a multi-tap transformer from one tap to another to supply needed currents and voltages within a preregulated voltage range in response to varying input line voltage and/or system load current and voltage demand requirements.
2. Description of the Related Art
The present invention finds immediate application in such an area as thermal printer power supplies.
Thermal printers usually print dot-matrix characters a line at a time across an entire page. In general, printing of the characters (usually in an n.times.m dot matrix) is accomplished by scrolling a sheet of paper under a printing bar having a plurality of heating elements. A character generator controls the individual heating elements selectively turning them on and off to form the desired characters a line at a time by thermal interaction with the scrolling paper. As is readily seen, in printing an ordinary page the current demands of the printing bar will vary depending on whether a blank line is being printed, requiring no printing elements and little to no current, to a solid underline, requiring all of the printing elements and the greatest current demand.
As printing (scrolling) speeds increase, the printer power supply must be able to quickly respond to the widely varying current demands of the printing bar. For a thermal power supply then to be efficient, it must provide only the minimum required power demanded by the system at any one time.
The present invention improves and differs from the known prior art as outlined below.
U.S. Pat. No. 4,090,234 to Goodman discloses a circuit for controlling a transformer secondary having a plurality of taps responsive to wide variations in input voltage.
U.S. Pat. No. 3,921,059 to Birman provides an automatically regulated power supply operated with a precision series regulator over a pre-determined range of output voltages or currents. This regulated range of voltages and currents is added to relatively large steps of unregulated voltage to provide an over all range of voltages and currents several times the pre-determined range.
U.S. Pat. No. 3,725,771 to Gilmore provides a control for a step voltage regulator having a reversible motor for actuating a tap changer switch to vary the regualtor output voltage whenever a "raise" or a "lower" signal is applied thereto.
U.S. Pat. No. 3,418,563 to Grosu provides a single-phase transformer having a switch for selectively connecting the secondary winding of the transformer and pre-selected pairs of winding taps to the outlet circuit of the transformer.
U.S. Pat. No. 3,217,230 to Hart discloses a voltage make-up regulator. The heart of this regulator is a make-up circuit employing a center tapped auto-transformer. A voltage to be controlled is applied to the center tap of the transformer and, through diodes spaced equally on opposite sides of the center tap, to a load circuit through a smoothing filter. Alternate switching pulses, the width of which are controlled by the magnitude of the voltage to be made up, are applied to normally open switches connected to opposite ends of the transformer only when the load voltage decreases below the desired value. The alternate closing of the switches results in current flow through alternate halves of the transformer winding and thus causing a voltage to be induced in the opposite half of the winding. This induced voltage is combined with the source voltage and results in the voltage applied to the load circuit being maintained at its previous level even though the source voltage decreases.
U.S. Pat. No. 4,531,134 to Horlander discloses a number of electrodes which are driven by an operational amplifier under control of a reference current source. The output of the operational amplifier and the magnitude of each of a number of resistors is selected so that the nominal voltage on a line is reduced by one-half across each of the resistors. This approximates a constant power output to a ribbon. The voltage regulation and constant power output each act to limit power dissipation at the ribbon surface, thus reducing debris at the printhead.
The present invention not only improves on the known prior art as outlined above, but also provides a device that can have a small, highly compact power supply with a simple instead of complex control circuit. These latter attributes are essential in allowing thermal printers, desirable for their low noise operation, to be used in areas having limited space, such as aircraft cockpits that also require highly power efficient devices.